Portable electronic devices, such as mobile telephones and ultra portable computing devices, frequently use batteries to power them. At the time of writing such devices typically use 3.6 volt rechargeable batteries. However these batteries have a start of life voltage of up to 4.2 volts and, if the user removes the battery whilst keeping the battery charger connected to the device then the voltage provided by some poorly regulated chargers can increase to 5.5 volts or so. Therefore a voltage regulator is provided between the battery and the circuits that it powers in order to ensure that these circuits see a nominally constant voltage. These circuits are often digital circuits and are commonly fabricated using sub-micron CMOS integrated circuit manufacturing technology which often has a maximum supply voltage of 3.6 volts or lower.
Whilst an additional circuit may be provided to act as the low drop out voltage regulator, and the additional circuit could use transistors formed in a different technology, it would be advantageous if the voltage regulator could be implemented on the same semi-conductor die as the CMOS integrated circuits.